1. Field of the Invention
The present invention relates to an electromechanical semiconductor transducer which converts mechanical strain into electrical signals.
2. Description of the Prior Art
The fact that a semiconducting material undergoes a change in electrical resistance when subjected to mechanical strain or stress is known as the piezoresistance effect. A silicon-based transducer which converts mechanical strain into electrical signals using this effect is being developed. The fundamental structure of such a transducer is shown in FIGS. 12 to 14. The transducer (6) shown in FIG. 12 is of a cantilever type and the transducer (7) shown in FIG. 13 is of a diaphragm type. The important strain gauge region is enclosed by line XIV, an enlarged view of which is shown in FIG. 14. The silicon-based electromechanical semiconductor transducer shown in FIG. 14 will be described in more detail below.
Usually the gauge region is constructed in the following manner. First, an n-type silicon substrate 1 is coated with an oxide film 2 (such as SiO.sub.2) and a p-type diffusion layer 3 is formed by the thermal diffusion of boron or the like. Subsequently, metal electrodes (ohmic electrodes) 4 are formed by the vacuum evaporation of aluminum or the like. Finally, the silicon substrate 1 and the metal electrodes 4 are entirely covered with a passivation film 5 of SiO.sub.2 or SiN.sub.x.
When a mechanical strain or stress is applied to the p-type semiconductor transducer mentioned above in the direction of arrow shown in FIGS. 12 and 13, the electrical resistance of the p-type layer changes. This change is linearly proportional to the mechanical strain or stress applied. Therefore, it is possible to obtain the mechanical strain or stress applied to this part by measuring a change in the electrical resistance.
However, the conventional silicon-based electromechanical semiconductor transducer has many disadvantages, some of them are described below.
First, the gauge region (p-type) is electrically isolated from the substrate (n-type silicon) by means of the reverse bias effect of the p-n junction. Because of this structure, the gauge region does not work satisfactorily at high temperatures. In other words, because the leak current increases at the p-n junction at temperatures above 150.degree. C., it is very difficult to electrically isolate the gauge region.
Second, in the case of silicon, it is difficult to form the hetero junction with crystal lattice matching for dissimilar semiconductors. It is only possible to utilize the piezoresistance effect of the conductive monolayer (p-layer). This limits the improvement of sensitivity and the optimization of element characteristics.
In addition, the energy forbidden band of silicon is narrower than that of compound semiconductors such as GaAs and Al.sub.x Ga.sub.(1-x) As (0&lt;.times..ltoreq.1). This causes the electrical conductivity to greatly fluctuate depending on temperatures. As a result, piezoresistance coefficient greatly fluctuates depending on temperatures.
Furthermore, the conductive substrate (n-type) needs a passivation film that entirely covers the external surface of the gauge. This poses a problem when the gauge is applied in the field of medical electronics.
Lastly, the formation of a p-type gauge region on the n-type silicon substrate requires many steps for SiO.sub.2 film formation (as a diffusion mask), as well as impurity diffusion, annealing and passivation film formation.